The present invention relates to a process for preparing a USP grade white oil by filtering through a specified clay sorbent.
A white oil produced in the present process meets the requirements of regulation 21 CFR 172.878, 21 CFR 178.3620(a), 21 CFR 178.3620(b) or 21 CFR 178.3620(c) for USP and technical grade white oils. White mineral oil is prepared from a distillate of petroleum crude oil which has a viscosity in the lubricating oil range and a normal boiling point above 350.degree. F. Preparation of the white oil generally includes one or more upgrading steps for purifying the oil and for removing the contaminants which degrade the properties of the oil as a white oil. Common upgrading steps include refining processing such as hydrotreating, hydrogenation, filtering, solvent refining and/or dewaxing. A final step for removing the last traces of contaminants may include one or more hydrogenation reaction steps, an acid treatment step and/or a final filtering step. The traditional method for making white mineral oil has been treatment of the distillate with acid. Traditionally, the distillate was treated with acid, and purified by clay treatment to remove by-products that have been created by the acid treatment. U.S. Pat. No. 4,024,026 and U.S. Pat. No. 4,053,367 disclose removing impurities, such as olefinic materials, from hydrocarbons by clay treating. Common contact materials for clay treating are acidic aluminosilicates, either naturally occurring materials, such as bauxite or mordenite clay, or a synthetic material and may comprise alumina, silica, magnesia or zirconia or some other compound which exhibits similar properties. Typical clays which are taught as being suitable include Filtrol 24, Filtrol 25, Filtrol 62, Attapulgus clay and TONSIL clay.
White mineral oils have also been prepared using hydrogenation processes. U.S. Pat. No. 3,459,656 to Rausch discloses a process for preparing technical grade and food grade white mineral oils by catalytic hydrogenation in two steps. The catalyst of the first hydrogenation operation is taught as being any of the sulfur resistant non-precious metal hydrogenation catalysts. The catalyst of the second hydrogenation is a platinum group metal-promoted catalyst. U.S. Pat. No. 5,294,327 discloses three stages of hydroprocessing without any solvent extraction or acid treatment prior steps to produce a desired food grade quality white mineral oil.
While a hydrogenation process may produce acceptable white oil during normal operation, it may produce "off-spec" product as a result of, for example, a process upset or a feed change. Such off-spec product may then require a second treatment in order to pass all of the requirements of a USP grade white oil, at additional expense and process complexity. U.S. Pat. No. 5,098,556 partly addresses the issue of off-spec oil by disclosing a process for removing the RCS present in off-spec white oil produced by catalytic hydrogenation by contacting the off-spec oil with a bleaching clay, such as a smectite clay, particularly an acid-treated montmorillonite clay, such as bentonite or attapulgite. According to the '556 patent, a white oil having an RCS value of approximately 2.5-3.5 is considered off specification, but treatable by clay purification. A white oil with an RCS value of 4 or over generally was found to be untreatable. Clay treatment of the white oil reduced the RCS value to under 2.5. The '556 patent does not clearly explain the number value attributed to the RCS test.
The determination of readily carbonizable substances (RCS) in a white mineral oil is described in ASTM D565-88 (Reapproved 1993). The test includes extracting a white oil sample with H.sub.2 SO.sub.4, and evaluating the color of the oil layer and acid layer following extraction. A white oil sample is considered as having passed the RCS test when the oil layer shows no change in color and when the acid extract layer is no darker than the reference standard colorimetric solution. However, while not required by the test, the acid extract layer may be further analyzed for color, using, for example, the ASTM Color Scale (ASTM D1500-96) or the Gardner Color Scale (ASTM D1544-80: reapproved 1989). As used herein, the term "RCS value" refers to the color of the acid extract from the acid extract layer. Unless otherwise indicated, the color is referenced to the ASTM color scale, and is indicated by a number per the ASTM color scale.
The '556 process is limited to white oils having an RCS value of 2.5-3.5. However, conditions beyond the control of the refiner may result in production of a more severely contaminated white oil. It is desirable to have an inexpensive and efficient process for upgrading such more severely contaminated off-spec oils without having to resort to additional hydrogenation or to other more costly and difficult methods.